Microarray analysis of gene expression following the formalin test in the infant rat☆

Effects of COX inhibition and LPS on formalin induced pain in the infant rat

In the adult, immune and neural processes jointly modulate pain. During development, both are in transition and little is known about the role that the immune system plays in pain processing in infants and children. The objective of this study was to determine if inhibition or augmentation of the immune system would alter pain processing in the infant rat, as it does in the adult. In Experiment 1, rat pups aged 3, 10, or 21 (PN3, PN10, and PN21) days of age were pretreated with NS398 (selective cyclooxygenase (COX)-2 inhibitor) or SC560 (selective COX-1 inhibitor) and tested in the intraplantar formalin test to assess effects of COX inhibition on nociception. Neither drug had an effect on the behavioral response at PN3 or PN10 pups but both drugs attenuated nociceptive scores in PN21 pups. cFos expression in the spinal cord likewise was reduced only at PN21. In Experiment 2, pups were injected with lipopolysaccharide (LPS) prior to the formalin test at PN3 or PN21. LPS increased the nociceptive response more robustly at PN21 than at PN3, while increasing cytokine mRNA equally at both ages. The augmentation of pain responding at PN21 was largely during the late stages of the formalin test, as reported in the adult. These data support previous findings demonstrating late maturing immune modulation of nociceptive behaviors.

Peripheral nerve graft with immunosuppression modifies gene expression in axotomized CNS neurons

Adult central nervous system (CNS) neurons do not regenerate severed axons unaided but may regenerate axons into apposed predegenerated peripheral nerve grafts (PNGs). We examined gene expression by using microarray technology in laser-dissected lateral vestibular (LV) neurons whose axons were severed by a lateral hemisection at C3 (HX) and in lateral vestibular nucleus (LVN) neurons that were hemisected at C3 and that received immunosuppression with cyclosporine A (CsA) and a predegenerated PNG (termed I-PNG) into the lesion site. The results provide an expression analysis of temporal changes that occur in LVN neurons in nonregenerative and potentially regenerative states and over a period of 42 days. Axotomy alone resulted in a prolonged change in regulation of probe sets, with more being upregulated than downregulated. Apposition of a PNG with immunosuppression muted gene expression overall. Axotomized neurons (HX) upregulated genes commonly associated with axonal growth, whereas axotomized neurons whose axons were apposed to the PNG showed diminished expression of many of these genes but greater expression of genes related to energy production. The results suggest that axotomized LVN neurons express many genes thought to be associated with regeneration to a greater extent than LVN neurons that are apposed to a PNG. Thus the LVN neurons remain in a regenerative state following axotomy but the conditions provided by the I-PNG appear to be neuroprotective, preserving or enhancing mitochondrial activity, which may provide required energy for regeneration. We speculate that the graft also enables sufficient axonal synthesis of cytoskeletal components to allow axonal growth without marked increase in expression of genes normally associated with regeneration.

Patterns of pain: meta-analysis of microarray studies of pain

Existing microarray gene expression profiling studies of tonic/chronic pain were subjected to meta-analysis to identify genes found to be regulated by these pain states in multiple, independent experiments. Twenty studies published from 2002 to 2008 were identified, describing the statistically significant regulation of 2254 genes. Of those, a total of 79 genes were found to be statistically significant "hits" in 4 or more independent microarray experiments, corresponding to a conservative P<0.01 overall. Gene ontology-based functional annotation clustering analyses revealed strong evidence for regulation of immune-related genes in pain states. A multi-gene quantitative real-time polymerase chain reaction experiment was run on dorsal root ganglion (DRG) and spinal cord tissue from rats and mice given nerve (sciatic chronic constriction; CCI) or inflammatory (complete Freund's adjuvant) injury. We independently confirmed the regulation of 43 of these genes in the rat-CCI-DRG condition; the genetic correlates in all other conditions were largely and, in some cases, strikingly, independent. However, a handful of genes were identified whose regulation bridged etiology, anatomical locus, and/or species. Most notable among these were Reg3b (regenerating islet-derived 3 beta; pancreatitis-associated protein) and Ccl2 (chemokine [C-C motif] ligand 2), which were significantly upregulated in every condition in the rat.

The beta3 subunit of the Na+,K+-ATPase mediates variable nociceptive sensitivity in the formalin test

It is widely appreciated that there is significant inter-individual variability in pain sensitivity, yet only a handful of contributing genetic variants have been identified. Computational genetic mapping and quantitative trait locus analysis suggested that variation within the gene coding for the beta3 subunit of the Na+,K+-ATPase pump (Atp1b3) contributes to inter-strain differences in the early phase formalin pain behavior. Significant strain differences in Atp1b3 gene expression, beta3 protein expression, and biophysical properties of the Na+,K+ pump in dorsal root ganglia neurons from resistant (A/J) and sensitive (C57BL/6J) mouse strains supported the genetic prediction. Furthermore, in vivo siRNA knockdown of the beta3 subunit produced strain-specific changes in the early phase pain response, completely rescuing the strain difference. These findings indicate that the beta3 subunit of the Na+,K+-ATPase is a novel determinant of nociceptive sensitivity and further supports the notion that pain variability genes can have very selective effects on individual pain modalities.

A novel automated method for measuring the effect of analgesics on formalin-evoked licking behavior in rats

The behavioral assessment of pain is essential for the analysis of pain mechanisms and the evaluation of analgesic drugs. The formalin test is one of such methods widely used as a model of injury-induced pain in rodents. This test is manually demanding and the recording of results is left to the subjectivity of the experimenters. Thus we developed a novel automated method to estimate the pharmacological response in formalin-induced licking behavior in rats using a multicolor detection technique. Two color markers were preliminarily applied to rats-yellow dye on the mouth and fluorescent green tape on the right hind paw. Behaviors of the animals were recorded from both above and below the subject, by a dual-view digital video camera system. After injection with formalin into the hind paw, rats exhibited a biphasic display of licking behavior. Licking time was measured by the sum of frames where the distance between these markers was less than an appropriate threshold of distance (TD). The split-plot analysis of variance demonstrated that the sum of squares of differences in licking time between manual and automated measurement was minimized when TD = 20mm. In addition, frames in which moving velocity of these markers is less than 2.5mm/s was neglected for calculation in order to eliminate sedative effect on the recorded data. On these conditions, subcutaneous administration of morphine in rats dose-dependently decreased formalin-elicited nociceptive responses. These results suggest that under optimal conditions the automated technique when applied to pharmacological studies are more reliable and efficient than if they are manually recorded.

Behavioral responses to pain are heightened after clustered care in preterm infants born between 30 and 32 weeks gestational age

OBJECTIVE

To compare biobehavioral pain responses of preterm infants born at differing gestational ages (GAs) when pain was preceded by a rest period or by a series of routine nursing interventions.

METHODS

In a randomized, within subjects, cross-over design, facial (Neonatal Facial Coding System), sleep/wake state and heart rate (HR) responses of 43 preterm infants [mean birth weight: 1303 g (range 590 g to 2345 g); mean GA at birth: 30 weeks (range 25 to 32)] were examined across 3 phases of blood collection (Baseline, Lance, and Recovery) under 2 conditions: pain after a 30-minute rest period versus pain after a series of routine nursing interventions (clustered care). Infant behavioral responses were coded from continuous bedside videotapes. HR was analyzed using custom physiologic signal processing software.

RESULTS

Infants born at earlier GA (<30 wk) had equally intense facial responses during the Lance phase regardless of condition. However, later born infants (> or =30 wk GA) showed heightened facial responses indicative of sensitized responses during blood collection when it was preceded by clustered care (P=0.05). Moreover, later born infants had significantly lower facial (P=0.05) and HR (P=0.04) reactivity during Recovery when blood collection followed clustered care.

DISCUSSION

Earlier born preterm infants showed heightened states of arousal and poor ability to modulate HR during Recovery when an invasive procedure was preceded by routine tactile nursing procedures. Alternatively, later born infants exhibited sensitized responses when clustered care preceded blood collection. Our findings support the importance of cue based individualized approaches to care.